The present invention relates to the production of a ternary or quaternary polycrystalline alloy to be used in the production of semiconductor compounds by passage in per se known manner into a solvent zone. More specifically, the invention relates to the production of a "source" alloy, which is intended, through passing into the solvent zone, to produce crystals of semiconductor compounds, which are either ternary of type Cd.sub.x Hg.sub.0.5-x Te.sub.0.5, or quaternary of type Cd.sub.x Hg.sub.0.5-x Te.sub.y Se.sub.0.5-y, in which 0&lt;x&lt;0.5 and 0&lt;y&lt;0.5. These compounds have a forbidden band width varying continuously with the atomic fractions x and y, which consequently permits the production of infrared photo conductors at any wavelength above 0.8 micrometre.
From a general standpoint, the use of a solvent makes it possible to produce such semiconductor compounds at a temperature below their melting temperature, so that it is possible to obtain purer materials with a better crystalline quality. More specifically, the use of a solvent in the case of semiconductor compounds Cd.sub.x Hg.sub.0.5-x Te.sub.0.5 and Cd.sub.x Hg.sub.0.5-x Te.sub.y Se.sub.0.5-y makes it possible to overcome the difficulties linked with the fact that mercury vapour has a high pressure at the melting temperature of such compounds.
The general principle of the method for forming a monocrystal by solvent transfer consists of making an ingot of a solid semiconductor compound passed longituninally through a molten zone of said solvent. This method is widely known and forms the subject matter of French Patents Nos. EN 74 42 769 of 24/12/1974 and EN 82 20 053 of 30/11/1982 filed in the name of the COMMISSARIAT A L'ENERGIE ATOMIQUE.
French Patent No. EN 74 42 769 describes the use of juxtaposed alloy bars as the materials constituting the supply "source" of the solvent zone for the growth of types II-VI semiconductor compounds. French Patent No. EN 81 05 387 of the Societe Anonyme de Telecommunications associates the operation of juxtaposing CdTe and HgTe bars with the use of an initial solvent zone constituted by a tellurium rich CdHgTe mixture. In this case, the use of a HgTe bar implies a limitation of the temperature of the solvent zone, which must not exceed 760.degree. C., which is the melting temperature of the alloy HgTe.
French Patent No. EN 82 20 053 proposes a method in which the supply material of "source" of the solvent zone is a compacted, interdiffused, homogeneous mixture of CdTe and HgTe mixture. This makes it possible to avoid the difficult machining of alloy bars, which is otherwise necessary for obtaining the desired exact proportions and makes it possible to overcome the preferential solubility of HgTe in the solvent. This method also makes it possible to carry out the transfer through the solvent zone at temperatures above 670.degree. C., but the residual porosity of the source material continues to be a handicap for producing long crystals.
In general terms, it can be seen that the various prior art processes lead to significant problems in connection with the preparation of ternary or quaternary crystals Cd.sub.x Hg.sub.0.5-x Te.sub.0.5 or Cd.sub.x Hg.sub.0.5-x Te.sub.y Se.sub.0.5-y when using the method of transferring a source alloy into a solvent zone. Thus, the preparation of a source alloy with a very considerable composition homogeneity and which is free from porosity is relatively difficult.
Ternary or quaternary alloys have already been produced by heating the compounds to the melting point in an ampoule sealed by a plug which may or may not be extended by a capillary tube, said ampoule being placed in an enclosure making it possible to raise the alloy temperature during formation, by placing said enclosure in an electric furnace and by maintaining in its interior, i.e. around the ampoule, a high pressure of a neutral or reducing gas, so as to compensate the pressure of the mercury vapour in equilibrium with the liquid to prevent it from escaping from the ampoule, or to ensure that the thin walls of the latter are not subject to a significant pressure difference between the inside and the outside.
These methods have been described in various articles by J. STEININGER, e.g. in Journal of Crystal Growth, vol. 37, pp 107-115, 1977 and by A. W. VERE et al, Journal of Crystal Growth, vol. 59, pp 121-129, 1982, but they lead to ingots having an inadequate homogeneity for the growth of a crystal by solvent zone passage.